The present invention relates generally to a magnetron sputtering apparatus, and more particularly to rotating magnetron cathodes with a spring-loaded mount.
DC reactive sputtering is a most frequently used deposition process for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazing. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. This system may be referred to as a continuous in-line system or a glass coater.
Inside the vacuum chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about 3 millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. A layer of material to be sputtered covers the cathode surface. This surface layer is known as the target or the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film. It is advantageous to produce the gas discharge in the presence of a magnetic field using an apparatus known as a magnetron with an array of magnets mounted in a fixed position behind the target and the magnetic field causing electrons from the discharge to be trapped in the field and to travel in a spiral pattern, thereby creating a more intense ionization and higher sputtering rates.
The rotary or rotating magnetron, developed to overcome some of the problems inherent in the so-called planar magnetron, uses a substantially cylindrical cathode and target. The target is rotated continually over a magnet array which defines a sputtering zone, such that a new portion of the target is continually presented to the sputtering zone. This has the favorable effects of easing cooling problems and allowing higher operating powers. The rotation of the target also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetron requires bearings to permit rotation, and vacuum seals for the drive shaft, the electrical conduits and the cooling conduits. Vacuum and rotary water seals have been used to seal around the drive shaft and the conduits which extend between the coating chamber and the ambient environment. Various mounting, sealing and driving arrangements are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are also hereby incorporated by reference. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends, each end of the cathode being attached to a spindle held by bearings attached to a sputtering chamber sidewall.
The magnetron, however, may be supported at only one end, designated as the drive end, by a cantilever mount. The other end of the cathode may be referred to as the free end. A cantilever mounted magnetron usually includes a bearing housing containing a drive shaft, a rotary vacuum seal, and at least two bearings spaced along the drive shaft, one of which may function as a shaft seal. Cantilever-mounted magnetron removal does not require removal of a difficult to remove sidewall mounting structure to provide the necessary clearance, and cantilever-mounted magnetrons require only one rather than two rotary seals.
A rotating magnetron incorporating a cantilever mounted removable cathode detachable from its bearing assembly is described in U.S. Pat. No. 5,100,527, assigned to the assignee of the subject application, the entire disclosure of which is hereby incorporated by reference. Such a system allows, among other things, the cathode to be removed easily and without special equipment, thus reducing system down time both by reducing the time required to replace a cathode and by making simultaneous removal of two or more cathodes more practical. A cantilever-mounted cathode having low vacuum seal loads is described in U.S. Pat. No. 5,200,049, assigned to the assignee of the subject application, the entire disclosure of which is hereby incorporated by reference.
In general, a dark space shield or sleeve may be concentrically disposed about the cathode body and spaced from its surface to form a gap. The distance across this gap is less than the dark space length. The dark space is the region of gas discharge next to the cathode. Here, the electrons accelerate under an applied operating voltage to become adequately energized to cause ionization of the sputtering gas. The dark space length is a function of the type of sputtering gas, the gas pressure and the applied electric field. The dark space length, for example, may be on the order of three millimeters.
The dark space shield, usually provided with means for attachment to a suitable mounting surface, protects a portion of the cathode body from the gas discharge and resultant ion bombardment. Dark space shields are usually provided at both ends of the cathode. The shield around the drive end of the cathode body should prevent the sputtering gas discharge from contacting that end. For the dark space shields used heretofore at the supported end or ends of the cathode, the mounting surface has been the chamber wall or a flange attached to the chamber wall, such that the dark space shield does not rotate with the cathode. The shield may also be electrically insulated from this mounting surface so that it is electrically isolated therefrom. The shield then floats electrically and acquires an electrical potential of the gas discharge. A preferred material for the shield is stainless steel.
During sputtering, a film of deposited material grows on the dark space shield, usually under tensile or compressive stress. The stress is highest on sharp edges. Eventually the film deposited begins to spall off, beginning usually on such sharp edges and on areas where the film is thickest. If the resulting flakes of material fall onto a substrate, they obstruct deposition on the areas of the substrate that they cover, resulting in defective products. In order to minimize the rate of film growth on a given dark space shield surface, that surface should point in a direction as close as possible to the direction away from the sputtering target.
The spacing between the dark space shield and the cathode must be well controlled to be less than a dark space length and to ensure that the shield does not touch the cathode.
The cantilever mounting arrangement produces a high bearing load, especially if a heavy target material, such as niobium, is used. High bearing loads have a detrimental effect on bearing life. Relieving the load on the bearing using an outboard support must allow for radial displacement of the free end to avoid additional loads.
Accordingly, an object of the present invention is to provide a rotating magnetron cathode with ,one end adapted to be attached to a spindle and the other end adapted to be supported such that the force on the bearings can be reduced, while maintaining the advantages of cantilever-mounted cathodes.